As is well known in the fields of automotive and architectural glass, multi-layered coating systems play a vital role in providing desirable characteristics such as transmittance, emissivity, reflectivity, durability, color, and chemical resistance. Such coating systems may be conventionally created by the well known commercial process known as sputter coating whereby a variety of different materials including metals and non-metals in pure, oxide or nitride form are applied to a glass substrate. An example of one such layer used in coating systems is silicon nitride (Si.sub.3 N.sub.4). Silicon nitride is desirable as a coating for glass because it is durable and forms a transparent protective layer. The layer or layers of silicon nitride may be applied by sputter coating silicon from a silicon cathode target in the presence of nitrogen gas.
Sputter coating is an electric discharge process, conducted in a vacuum chamber and in the presence of one or more gases. The equipment employed normally includes a vacuum chamber, a power source, an anode, and one or more specially prepared cathode targets covered with the material used to create a layer in the coating system. This equipment is expensive, and techniques for producing sputter coated articles are complex, time consuming and must be carried out with great precision. Consequently, like many other industrial processes, once the process is started, it is desirable to allow the process to continue for as long a period of time as possible and to produce as much end product as possible.
When an electric potential is applied to the cathode target, the background gas forms a plasma that bombards the target causing particles of the coating material to be liberated. The liberated coating material then adheres to the substrate and other exposed surfaces within the vacuum chamber, such as the chamber walls and the anode. When conducted in the presence of a reactive gas, a reactive product of the coating material is deposited on the substrate, i.e. the coating layer is the product of the coating material and the reactive gas.
In commercial sputter-coating systems such as those produced by Airco Corporation, a cathode target typically has sufficient coating material on it to coat glass substrates uninterruptedly for up to seven to fourteen days. Depending on process parameters, some systems are designed to continue for up to as much as seventy days. Unfortunately, while a single cathode target may be designed with sufficient coating material so as to be capable of coating thousands of linear feet of glass, present sputter coating systems rarely continue uninterrupted for even three days. The reason for this is as follows:
When producing a coating which is conductive, such as a metal or certain reactive products of that metal, the sputter coating process can generally continue until the cathode target is exhausted. However, when, as often occurs, the coating material or its reactive product which must be used is an insulator, a semiconductor, or is otherwise a substantially electrically nonconductive material, a build-up of the nonconducting material on the anode causes a progressive slow-down and eventual stoppage of the coating process. In short, such a coating on the anode inhibits and eventually prevents charge carriers from flowing from the anode to the cathode, thus at first reducing and eventually virtually stopping the sputtering process. Exemplary of such nonconducting materials are the often highly desirable coating components of silicon (Si) and aluminum (Al) including their nitrides. In this respect silicon targets are often doped with about 5% aluminum, but as such merely manifest the same problem as targets made up only of one or the other of these two desirable materials.
In addition to process downtime, the build-up of substantially electrically nonconductive coatings on the anode of a sputter coating device may have several other adverse affects on the coating process and on the coating formed on the substrate. Nonuniformities may occur in the coating due to changes in the size of the conductive area of the anode. Furthermore, large pieces of coating material may drop off the cathode target and onto the substrate because of arcing at the cathode target, and flakes of material may fall off the anode because of the poor adhesion of the coating to the anode. Poor adhesion is characteristic of a thick build-up of a substantially electrically nonconductive coating.
When the anode becomes coated with an insulative material blocking the flow of charge carriers or creating coating nonuniformities, the coating process must be halted in order to clean or change the anode. This reconditioning includes venting the chamber, careful cleaning and reevacuating the chamber. In typical commercial sputter-coating techniques, such as for producing layer systems which include one or more layers of nonconductive silicon nitride, the process may often have to be stopped for reconditioning the anode after as little as two days of continuous use. Such reconditioning then often requires about 24 hours to complete. Thus, downtime may represent up to a third of the total available time for production, a very large and undesirable proportion of the available production time.
U.S. Pat. No. 4,478,700 to Criss proposes a solution to this problem by using multiple cathodes with each anode. While monitoring the bias voltage on the anode, a desired coating is sputtered onto the substrate and the anode until the bias voltage reaches a level at which coating efficiencies become unacceptable. At that time, a new cathode is introduced which sputters a different, but now conductive coating onto the anode and the substrate as well. This conductive coating is designed so as to serve no other useful purpose in the layering system. The conductive layer formed on the anode thereby serves to lower the bias voltage of the anode, increasing process efficiency without completely halting the sputtering process. In such a process, however, there is required a plurality of cathodes and a mechanism, such as a rotating table system, to present the various cathodes to the coating process. Furthermore, Criss produces alternating layers of the desired coating and of the conductive material on the substrate. This has the potential of eventually building up to unacceptable thicknesses.
Other solutions which have been proposed to solve the problem of non-conductive coatings on the anode include rotary anodes, anodes with very large surface areas, and finned anode configurations. In another system, two cathode targets are employed, each of which is capable of acting as both a cathode and an anode. The system is periodically reversed so that a coating built-up on the anode is sputtered off when operated as a cathode. In a hybrid of this latter system, two cathode targets are used and the sputter magnets are alternated between them so as to alternatively apply a nonconductive and a conductive layer on the anode.
The above known attempts to solve this anode coating problem present their own drawbacks in terms of expense, complexity and, in some instances, even commercial feasibility. Delicately dimensioned fin geometry, for example, is required in one of the above solutions, complex alternating electrical controls in another, and excessively large anodes in another.
In view of the above, it is apparent that there exists a need in the art for an improved sputter-coating target and method of using such target which overcomes the above-described problems as well as other problems which will become more apparent to the skilled artisan once given the following disclosure.